Method of producing fluosilicic acid of low phosphorus content from waste gases

ABSTRACT

The hot waste gases obtained from crude phosphate digestion in the manufacture of superphosphate were diluted with eight to 12 times their volume of air and the sludge droplets and the crude phosphate dust were then separated in a cyclon with a limiting particle size of at most 0.01 mm. before eluting the H2SiF6 with water.

I United States Patent [15] 3,640,685

Goller 1 Feb. 8, 1972 [54] METHOD OF PRODUCING [56] References CitedFLUOSILICIC ACID OF LOW UNITED STATES PATENTS PHOSPHORUS CONTENT FROM 3In 384 11/1963 H k m a! 23/ eca omet 153X WASTE GASES 3,246,977 4/1966Hinkle, Jr. ....23/l53 X [72] Inventor: Heinz Ge er, LinZ m D Austria2,369,791 2/1945 Moore ..23/88 3 219410 11/1965 Dexter ..23/153 [73]Asslgnee: Osterrerchnsche Stickstoffwerke Aktlengeseuschaft, Linz amDanube Austria 3,024,086 3/1962 Cmes ..23/153 X [22] Filed: Apr. 6, 1970Primary ExaminerEdwarcl Stern [21] PP Na: 26,165 Attomey-Wenderoth, Lind& Ponack [57] ABSTRACT [30] Foreign Apphumon Pnomy Data The hot wastegases obtained from crude phosphate digestion Apr. 8, 1969 Austria ..A3370/69 in he m nufacture of superphospha e were diluted with eight to12 times their volume of air and the sludge droplets and the [52] US. Cl..23/153, 23/88, 23/205 crude ph sph du were h n separated in a cyclonwith a [51] Int. Cl ..C0lb 7/00,C01b 33/08,C01b 7/22 limiting p cle s ofa most .01 mm. before eluting the [58] Field of Search ..23/153, 88,205, 182 H SiF with water.

2 Claims, No Drawings This invention relates to a simplified process forobtaining crude fluosilicic acid of particularly low phosphorus contentfrom the waste gases obtained from the digestion of crude phosphate whenmanufacturing superphosphate.

The gases produced on digesting crude phosphate with sulphuric acidinter alia contain, depending on the variety of crude phosphate, about20 g./m. of fluorine in the form of HF and SiF.,. The toxicity of bothcompounds in itself demands careful purification of the waste gases.Furthermore, the waste gases from a valuable raw material source forobtaining fluorine compounds.

The HF and the SiF are easily separated out by washing the waste gaseswith water in a suitable washer, whereby an aqueous solution offluosilicic acid (silicofluoric acid) is formed:

2HF+SiF +aq H SiF -aq This fluosilicic acid can be further processed ina known manner, for example to give sodium silicofluoride, aluminumfluoride or cryolite, Na AlF The crude fluosilicic acid obtained in thisway contains about 2.5 percent of P O relative to percent strength H SiFThe P originates from the sludge droplets of the material being digestedand from the crude phosphate dust, which are carried by the resultinggas into the washer. Such a high P 0 content interferes significantlywith the further processing of both the fluosilicic acid and itssubsequent products and must therefore be removed or, better still, beavoided from the start.

It has been found that condensation very simply can be avoided bydiluting the waste gases, before entering the cyclone, which may possessa limiting particle size of at most 0.01 mm., with air to the point thatthe temperature under no circumstances falls below their dew point.Experience has shown that this requires a dilution of the waste gaseswith 8 to 12 times their volume of air. The subsequent elution of the HSiF is not impaired thereby.

Accordingly the present invention provides a process for producingfluosilicic acid of particularly low phosphorus content from thewastegases obtained from crude phosphate digestion in the manufacture ofsuperphosphate which process comprises diluting the waste gases from thedigestion vessel for the crude phosphate, which are at a temperature of90 to 100 C. with 8 to 12 times their volume of air, separating thesludge droplets and the crude phosphate dust in a cyclone with alimiting particle size of at most 01 mm., calculated according to theformula set out herein and subsequently eluting the H SiF with water.

The original amount of the waste gases is to be calculated from thewater vapor pressure above the sulphuric acid used for the digestion,with an addition for HF and SiF,,.

The limiting particle size is to be calculated according to the formulaof Rosin, Rammler and lntelmann (z.VDl, 76( 1932), 433/437):

the diameter of the smallest particle which is still separated out,measured in meters.

W the mean speed ofcntry of the carrier gas into the cyclone in m./sec..r the distance of the dip tube from the outer wall of the cyclone inmeters D the diameter of the cyclone in meters.

Cyclones of the type of the VAN TONGEREN separator have provedparticularly suitable.

The sludge droplets separated off in the cyclone and the EXAMPLE Wastegases issuing at a temperature of 94 C. and in an amount of 900 to 1,000mflhour from a crude phosphate digestion vessel with an output of about28 tons/hour of superphosphate under the customary digestion conditions,gave 690 kg. of fluosilicic acid containing 22 percent by weight of HSiFhd 6 on absorption by water; this acid contained 5 g./1 of P 0 Afterpassing through a cyclone (twin cyclone of the type of the VAN TONGERENseparator) of 1.14 m. diameter and dilution of the waste gases with10,000 m./hour of air, an acid of the above concentration whichcontained only 35 mg./ 1 of P 0 was produced under otherwise identicalworking conditions.

The residue separated off in the cyclone was continuously returned tothe digestion vessel together with 180 l./h. of water.

What we claim is:

1. A process for producing fluosilicic acid from the waste gasesobtained from the crude phosphate digestion in the manufacture ofsuperphosphate, which process comprises diluting the waste gases evolvedfrom the digestion of the crude phosphate, which waste gases contain HF,SiF, and sludge droplets of the material being digested and crudephosphate dust, which gases are at a temperature of to C., with 8 to 12times by volume of air per volume of the waste gases, separating thesludge droplets and crude phosphate dust by passing said diluted wastegases into a cyclone having a limiting size of at most 0.01 mm. andsubsequently eluting the resulting waste gases with water to formfluosilicic acid, whereby said limiting particle size is calculated y inwhich X,,,;,. limiting particle size;

is defined as the diameter of the smallest particle which is stillseparated out, given in meters 1 is the dynamic viscosity of air atgiven temperature in kgJmrsec.

P is the density of the dust and of the droplets in kgJm.

W is the mean speed ofentry of the carrier gas into the cyclone inmJscc.

S is the distance of the dip tube from the outer wall of the cyclone inmeters and I) is the diameter of the cyclone in meters.

2. A process according to claim 1 in which the cyclone is rinsed withwater in order to remove the sludge droplets and the crude phosphatedust which are separated and the resulting suspension is returned to thedigestion vessel.

2. A process according to claim 1 in which the cyclone is rinsed withwater in order to remove the sludge droplets and the crude phosphatedust which are separated and the resulting suspension is returned to thedigestion vessel.